Pfizer Inc. (NYSE:PFE), presently the world’s largest seller of central nervous system (CNS) drugs, may have its future lead challenged by a sharp rise of the biotech giant, Biogen Inc (NASDAQ:BIIB), as per a recent report by GlobalData.

Pfizer led the global CNS drug sales in 2014, raking in almost $8.1 billion. The sales were mostly driven by the pharma giant’s blockbuster pain medication, Lyrica (pregabalin), which posted a 12% year-over-year (YoY) growth in sales, reaching $5.1 billion in 2014. Though impressive, the overall CNS sales figure for Pfizer demonstrates no upside compared to 2013.

In the meantime, Biogen CNS drug sales soared 46% YoY to reach $8 billion in 2014. The sharp spike in Biogen sales, mostly contributed by its expanding share in the multiple sclerosis (MS) market, carried the biotech giant from third to second position in global CNS drug sales.

Currently, Biogen leads the MS drug market in the US, with sales contributed in large part by an established MS treatment, Tysabri, as well as several new drug launches, including Tecfidera and Plegridy. The company posted MS sales amounting to $2.1 billion in its first-quarter fiscal year 2015 (1QFY15) earnings release late last month, up from $1.7 billion in the same period last year.

The imminent danger to Pfizer’s top position in CNS drug sales strikes another huge blow to the already-pressured giant. The company has been seeing declining sales since patent expiry of its top-selling cholesterol-fighter statin Lipitor in 2012. In its patented days, Lipitor used to generate almost $13 billion annually for Pfizer. The company further faces generic competition for other star drugs, particularly in the UK. US sales suffer just as badly, with generic copies almost wiping out sales from its once best-selling Alzheimer’s drug, Aricept. On top of that, Pfizer is set to lose several more drug patents starting 2017.

On the other hand, Novartis AG (ADR) (NYSE:NVS) saw its global CNS drug sales rise 7% YoY to $5.7 billion in 2014. Much of this sales rise is contributed by its MS drug portfolio, led by Gilenya, which generates over $2.5 billion in annual sales. Gilenya currently competes against Biogen’s Tecfidera. Despite an early market entry, the drug lags behind Tecfidera in sales owing to several safety risks that were discovered soon after its launch. Novartis recently trumped Pfizer as the world’s biggest drug-maker by revenue, moving along the CNS drug sales ladder to the third position.

Meanwhile, Japan-based Otsuka Pharmaceutical Co., Ltd. fell by a staggering 50% in CNS drug sales to $5.3 billion in 2014. The company once boasted the leading CNS drug in the world, anti-psychotic medicine Abilify. Abilify, sold in collaboration with Bristol-Myers Squibb Co (NYSE:BMY), is now reeling from lost sales due to consecutive patent losses. The drug generated nearly $4.4 billion in sales last year, down from $7.2 billion in 2013. Abilify sales are further expected to decline by a hefty $6.2 billion by 2019, owing to widespread market availability of generic copies.

Another victim of generics, Eli Lilly and Co (NYSE:LLY) saw its CNS drug sales share decline by 28% YoY to $3.5 billion in 2014. The major driver of Lilly’s CNS share growth, Zyprexa, started facing patent expiry in 2011. The bipolar and schizophrenia drug saw its sales plunge to $1 billion in 2014, from a 2010 high of more than $5 billion.